Nanopatterning on Nonplanar and Fragile Substrates with Ice ResistsAnpan Han,†,∥ Aaron Kuan,‡ Jene Golovchenko,†,‡ and Daniel Branton*,§

†Department of Physics, ‡School of Engineering and Applied Sciences, and §Department of Molecular and Cellular Biology, HarvardUniversity, Cambridge, Massachusetts 02138, United States

ABSTRACT: Electron beam (e-beam) lithography using polymer resists isan important technology that provides the spatial resolution needed fornanodevice fabrication. But it is often desirable to pattern nonplanarstructures on which polymeric resists cannot be reliably applied.Furthermore, fragile substrates, such as free-standing nanotubes or thinfilms, cannot tolerate the vigorous mechanical scrubbing procedures requiredto remove all residual traces of the polymer resist. Here we demonstrateseveral examples where e-beam lithography using an amorphous ice resisteliminates both of these difficulties and enables the fabrication of uniquenanoscale device structures in a process we call ice lithography.1,2 Wedemonstrate the fabrication of micro- and nanostructures on the tip of atomicforce microscope probes, microcantilevers, transmission electron microscopygrids, and suspended single-walled carbon nanotubes. Our results show that by using amorphous water ice as an e-beam resist, anew generation of nanodevice structures can be fabricated on nonplanar or fragile substrates.

KEYWORDS: Nonplanar, nanopatterning, e-beam lithography, ice lithography, resists

We consider first the fabrication of a rectangular metalliccap on the pyramidal tip of a standard atomic forcemicroscope (AFM) cantilever (Figure 1a). A cantilever wasloaded onto a cryostage in a field emission scanning electronmicroscope (SEM) that had been modified for ice lithography.3

The sample was cooled to

wide, 300 μm long wires of 30 nm thick Au on 1 nm thick Ti.The resulting gold film was reflective and showed nodiscoloration, and the lift-off was very clean over the entiresample. Much narrower metal lines can also be patterned onAFM cantilevers (Figure 2, inset); the narrowest metal lines wepatterned onto AFM cantilevers were 15 nm wide.

In separate experiments, we measured the room temperatureelectrical properties of ice lithography prepared Au and Pdwires that were 10 nm thick and 500 nm wide. The resistivitieswere respectively 150 and 500 nΩm, which is between 5 and 6times higher than bulk values. This is typical for very thin filmswith high surface and grain boundary scattering.9,10 In additionto low-melting point metals, we have also made nanostructureswith tantalum, which has a very high-melting point at 3017 °C.We can deposit almost any material with high purity by in situsputtering, whereas methods such as gas-assisted focusedelectron and ion beam deposition are limited by the choiceof materials and tend to incorporate contaminants that rangebetween 20 to 90% of the total deposited material.11

We also patterned the surface of fragile freestanding Si3N4membranes supported on a Si wafer frame shaped like anelectron microscope grid (Figure 3). Silicon nitride membranes

find important applications as platforms for plasmonicnanostructures12,13 and nanopores14 but are too fragile towithstand ultrasonication, which is usually used with polymericresists to assist lift-off. For example, in connection with aplasmonics research project we needed a thin silicon nitridemembrane bearing metallic structures that were separated fromeach other by a nanogap. Because two triangular shapes withclosely spaced apexes lend themselves to forming such gaps, wefabricated an array of variously shaped triangular pairs (Figure3b, inset). By simply patterning the ice with triangular shapesseparated from each other with the desired gap width, wecreated metallic structures with features less than 10 nm wide(Figure 3a) and achieved gaps less than 7 nm wide (Figure 3b).Note that the lift-offs were very clean with no undesired metalflakes. Characterization of these structures by transmission

Figure 1. Ice lithography to cap an AFM Si3N4 tip with gold. (a)Processes for ice lithography. Amorphous ice (cyan/white) iscondensed on all accessible surfaces of a nonplanar sample at

electron microscopy (TEM) and in situ qualitative X-rayphotoelectron spectroscopy showed no stray metallic materialin the gaps fabricated between the structures.Even the back side of the Si3N4 membranes, that are

accessible at the bottom of the 200-μm deep pits etchedthrough the Si wafer, was straightforward to pattern with icelithography (Figure 3c and d). It would not be possible to applya controlled, homogeneous spin-coated resist in such deep pits.The patterns shown in Figure 3 were on 60 nm thick Si3N4, butwe have also successfully patterned on membranes as thin as 20nm.Single-walled carbon nanotubes (SWCNT) are quasi one-

dimensional nanostructures that have been studied extensivelybecause of their superior physical properties and potentialapplications. But lithographically patterning freely suspendedSWCNT using spin-coated resists is not possible because suchfreely suspended SWCNT are both nonplanar and very fragile.We grew electrical insulating nanoparticles coaxially on freelysuspended nanotubes or small bundles of nanotubes using icelithography (Figure 4). The carbon nanotubes were first grown

over a 1 μm wide trench in a free-standing silicon nitridemembrane.2 As previously described, ice-covered nanotubescan be imaged through the ice in the SEM using electron dosesfour orders of magnitude smaller than that required for actualpatterning.2 We then removed several 25 nm long regions ofice, each ice-free region separated from the next by 150 nm(Figure 4a, inset). The white arrows show the locations whereice was removed. After writing and transfer to the metaldeposition side chamber, 5 Å of Ti was sputter deposited onthe sample to form an adherent coating on the ice-free regionsof the nanotubes.After removal from the ice lithography SEM and immersion

in isopropanol, the sample was dried by carbon dioxide criticalpoint drying to avoid violent interfacial forces. Subsequently,the sample was transferred through air into an atomic layerdeposition (ALD) chamber where 20 nm of Al2O3 was grownat 225 °C on to the free-standing nanotubes. ALD does notgrow on pristine SWCNTs2,15 but does of course grow onoxides. Upon exposure to air, the patterned Ti on thenanotubes forms a surface oxide that initiates patterned ALDgrowth on the nanotube. As a result, Al2O3 nanoparticles wereformed on the SWCNT. The overall geometry of thesenanoparticles, including their location, was precisely deter-mined by the ice lithography patterning and ALD processingparameters that deposited the Al2O3.Our results demonstrate the utility of ice lithography for

patterning and lift-off on nonplanar and fragile substrates andshow the adaptability of a simple water−ice resist in e-beamlithography. Since ice lithography can pattern silicon dioxidelines down to 5 nm wide1 and metal wires down to less than 10nm wide,3 it achieves as great or greater resolution than e-beamlithography using polymer resists. But because an ice resist iseasily removed, leaving no residue following a dip inisopropanol or any other volatile solvent, ice lithography canbe used to fabricate carbon nanotube field effect transistors andother fragile devices whose performance would be compro-mised by the residue of polymeric resists. Some polymericresists can be deposited on nonplanar surfaces,16 but theirreliable removal, especially from fragile substrates, is problem-atic. Although the critical electron dose for water ice removal isroughly three orders of magnitude larger than that required forthe typical exposure of a polymeric resist, such as polymethylmethacrylate,1 the low sputter yield for water (0.03 H2Omolecules ejected per incident 5 keV electron)1 will not be aserious concern in most situations where e-beam lithography isused for research or mask fabrication, rather than direct high-volume commercial device production. Furthermore, the lowsputter yield for water ice makes it possible to visualize and mapthe exact location of critical nanostructures through the icelayer without removing significant amounts of the overlyingice.2 Such through-resist mapping can be very advantageousbecause the desirable properties of many nanostructures, suchas carbon nanotubes, are easily compromised by contaminationor damage during unprotected direct exposure to the e-beam ofan electron microscope.2,17

Our demonstration that ice lithography enables thefabrication of nanostructures on nonplanar and fragilesubstrates opens the possibility of research with nanodeviceconfigurations that have previously been difficult or impossibleto obtain. These include devices with localized metal structureson CNT tips for high-resolution magnetic force microscopy,18

improved and simplified production of tip enhanced Ramanspectroscopy cantilevers,4 and metal enhanced fluorescence

Figure 4. Growing nanoparticles on SWCNTs. (a) TEM of Al2O3nanoparticles grown by ALD on a bundle of SWCNTs on which 5 ÅTi had previously been deposited during ice lithography. White arrowsindicate areas from which the ice had been removed by the e-beam;the purple bar shows the width of the ice-freed regions. Thenanoparticles are sufficiently electron transparent that the nanotubesare clearly visible even where they are coaxially coated by the Al2O3.Inset: SEM image of patterned ice resist on the nanotubes before Tideposition. (b) Schematic of the experiment (not to scale). A 100 pA,30 keV e-beam dose of 3 C/cm2 was used to pattern a 60 nm layer ofice.

Nano Letters Letter

dx.doi.org/10.1021/nl204198w | Nano Lett. 2012, 12, 1018−10211020

nanotubes.19 In addition, ice lithography enables many othersensors and devices that are dependent on materials, such asgraphene or even biological components, whose performancecan be undesirably altered by e-beam exposure damage duringfabrication, organic solvents during resist development, or byunwanted residues of polymeric resists.Methods. All of the ice lithography steps and SEM imaging

were performed in a modified field emission scanning and e-beam writing JEOL JSM-7001F SEM, as previously de-scribed.2,3 The modifications of the SEM include a cryogeni-cally insulated stage that could be cooled to